The present invention relates to a recycling process that converts spent acid from a variety of industrial processes into useful commercially valuable products, including magnesium sulfate. In particular, my process converts spent sulfuric acid that can be classified as a RCRA hazard waste, into marketable products, including, smeltable metal oxides and magnesium sulfate.
Sulfuric acid is used in hundreds of industrial applications from metal finishing to food production. Some uses include pickling processes, oil refining, fertilizer production, pulp and paper production, and in the production of printed circuit boards, semiconductors and silicon wafers. In fact, sulfuric acid is the world's mot widely used chemical with U.S. consumption exceeding 38 million metric tons annually. Most, if not all, of these uses of sulfuric acid generate large volumes of spent waste sulfuric acid containing all types of inorganic and organic contaminants.
Typically, waste sulfuric acid is disposed of by a regeneration process that involves spraying sulfuric acid into a combustion furnace to generate sulfur dioxide (SO2). This SO2 is then converted to sulfur trioxide (SO3) and eventually absorbed in 98% sulfuric acid to obtain 99+% pure sulfuric acid. These processes are disclosed in U.S. Pat. No. 6,399,040. Obviously, a major drawback of such disposal processes is the large amount of energy needed and the cost associated with the thermal decomposition.
Other known disposal processes involve the use of CaCO3 or CaO treatment to neutralize the acid and form useable gypsum (CaSO4.2(H2O)). Gypsum is used in cement, dry wall, plaster and a number of other products. One process using CaCO3 in a scrubber, as disclosed in U.S. Pat. No. 6,099,814, produces a secondary product of magnesium sulfate. The problem with these types of processes is that mountains of gypsum are produced that exceed the demand for such a product. Such processes, however, are limited in that the waste acid that can be used cannot contain metal contaminants that are typically found in waste acid streams, such as, copper, iron, lead and other metals that precipitate out with the gypsum, and thus lead to a low or no value product. In yet another disposal process spent sulfuric acid is used to form iron oxide pigments (see U.S. Pat. No. 6,530,987).
Because such large volumes of hazardous spent waste acids like sulfuric acid are generated every year, a need exists for a simple and efficient recycling process to convert these spent waste acid streams into useful and consumable products. My process accomplishes this goal as will become evident from the following specification and claims.